Amplitude Response Audio Amplifier

The report shows the
measuring technique of the amplitude response for an amplifier. A software
supported measurement in connection with precision circuit analyzers and a
sine level generator is presented.

What means amplitude response?

Short Introduction

An amplifier has the task the voltage applied at
the input in such a way to amplify that the voltage at the output has the
accurate same process. The user has in the hand whether the voltage applied
at the input is to appear multiplied by a factor at the output. This
adjustable factor is the volume control. It is of elementary importance that
this multiplying factor remains always with all input voltages, frequencies
and loads the same.
Amplitude Response
The amplitude response shows, the multiplying factor is how constant over
the frequency? There are many representation possibilities, the fantasy is
set no borders. A function representation is usual in the XY coordinate
system, whose axles are however often also linear shown logarithmically to
the basis 10. The professional among the readers, asks now immediately for
the phase response. Correctly, both are two inseparable brothers and
sisters. It is forgiven me that the phase response is measured with another
measuring system and treated therein separately.
Bandwidth what is that?
Is in principle a simple affair, which is mathematically fast avowed. It
is with an amplifier a useful characteristic, but something misleading, in
particular for a layman.
The range is completely primitively said a guideline for the speed of an
amplifier,: the more highly the range the faster. The range is recognizable
from an amplitude response. It is a size, which is often wrongly regarded in
purchase to an amplifier. The range shows the point in the amplitude
response, at which the output voltage dropped to approximately 70.7%
(1/square root 2) to target the output voltage. Now the large matter of
price: would you like to use an amplifier with a frequency, with which the
amplifier causes already 100%-70.7% = 29.3% errors? Assumed the range
amounts to only 20 kHz, such an amplifier is how good?
- Bad - you can save your time to read a test report. To power on
mesurement devices for something like that - no - only you want to see the
proof, it's always the same lyre. Therefore all measuring instruments to
start is pure waste of time, it is one wants exactly negatives to confirm,
what one can already foresee. - Yes, - the defender of its noble pieces
comes now with this popular argument: the human hearing can hear only about
16 kHz -20 kHz. There say I nevertheless easily against it 29.3% error with
30 kHz. With 20 kHz there is then already too much (is too idle me to
calculate because of critics for 20 kHz and a low-pass of certain order). A
lameness in the amplifier draws a tremendous chain of other negative effects
behind itself ago. Which correctly decent begins with approximately 100 kHz
range and at the same time high open loop under full load conditions. Not
only fast by high range, but also precisely by the high open loop gain.

The
pictures show the results of the system from two HP3457A (here 6.5 places)
and a HP3336B frequency Sythesizer. The measurement happened in a friendly
calibration laboratory, which put the devices to me at the disposal. In
addition I wrote an automatic program sequence under utilization of the
GPIB interfaces.

Measurement shows the ganging of both meters connected to the same signal
source.

The
stability of the signal source is fine, but not essential as long as the
meters doing their job satisfactory. The 3336 together with the option 05
(precision amplitude attenuator) has an amazing constant amplitude. The
decreased amplitude at higher frequencies has his origin in the amplitude
response of the meter itself. And of course may be also from the generator
(I don't think so much) or the cables or a marginal wrong termination.
It's not cleary why, but for audio applications inconsiderable.

The
picture left shows the measurement consists of two HP3457A and
a generator HP3336B.
The cables are connected like a mirror and terminated with 75 ohm. For
best results the resistor should be on the end of the cable, it's
difficult here to do it without an special soldered shared construction,
so I missed to do it, sorry. Note the frequency is adjusted to 1 MHz.

How to get a higher accuracy?

In particular to settle the claim of "Ultra" there is
some additional work to do.

a fresh calibrated instruments (constant 0.003 dB could be reduced)

same temperature during calibration and measurement, as possible measure
direct after calibratiion

use for both meter same cable types and length and manufactorer

as possible ideal refexion free termination of the generator impedance

calibrate over many points as possible, put the values in a computer, do a
fit function and use this function to correct future measured values

set instrument resolution to 7 1/2 digits (leds to a long measurement
time)

do many measurements at same frequency and calculate mean value

More
handling ways for a better accuracy:

if you have the change use a HP3458A meter (up to 8 1/2 digits). The 3457A
is already one of the best meter HP builts in the past, but the 3458A that's
really taking "the biscuit of accuracy". Both meters having a high accuracy.

Ask the calibration laboratory particularly conscientiously to work. The
laboratory is to measure as much as possible points, in order to produce a
family of characteristics, which can be out-corrected computationally. Thus
the accuracy is close at the standard.

Does
it make sense to do this stress of an improved accuracy because of audio
measurements?

no

the measurement time should be not too long, the device under test (audio
amplifier) will drift with his amplitude response vs. time and room
temperature certainly more than the accuracy of the measurement system.

for audio amplifier (also the best ones) is this system already very good
with enough room for accuracy.

The graphs
showing details of some measurement ranges. Most meters having their
highest precision under a maximum utilisation of their range. But these
instruments still have a good accuracy even under a low range utilisation.

Nice to see the meter works fine in any range.

These are the voltages applied to the input meter

Voltages applied on the output meter

Measurement of the 3 volts range. Good result under any applied voltage

Voltages applied to the input meter

Voltages applied to the output meter

Shows the results of the 30mV measurement range

Voltages applied to the input meter

Voltages applied to the output meter

What will happens using the 30V range together with the 300mV range?

can be
foreseen - very similar results. So far however yet tested, does not need a
frequency-compensated precision voltage divider plus GPIB capable generator for
higher constant tensions plus much time. Since I can predict the behavior for
such a combination of measured the so far, I would like to do without such a
measurement, even if such a combination for audio is meaningful applications.
The accuracy reserves are still very well enough for Hifi applications.

For what
I'am using this system?

well
clearly, of course to measure the amplitude response of hifi amplifier.